1. Field of the Invention
The present invention generally relates to luminaires of the type having a high pressure sodium lamp, an electrical power source of voltage and current, and ballast-starter means operatively connected between the power source and the lamp, and normally operative for supplying voltage and current at a predetermined rated power factor across the lamp-ballast-starter combination to light the lamp during nighttime conditions. More particularly, the present invention relates to a power factor measuring cut-off arrangement for and method of protecting the ballast-starter means from damage in the event of a cycling malfunction, wherein the lamp is alternately on and off, and wherein the power source supplies voltage and current to the lamp-ballast-starter combination at a power factor different from said predetermined power factor. A method of installing the power factor measuring cut-off arrangement is also disclosed.
2. Description of the Prior Art
Luminaires having a high pressure sodium lamp and a ballast-starter circuit operatively connected between an electrical power source and the lamp to operate the latter are well known, particularly for streetlighting purposes. The operating characteristics of this sodium lamp are such that, as the lamp ages, some of the electrode material will deposit on the arc tube. This causes the arc tube to retain heat and, in turn, the internal pressure and the arc tube voltage will increase. When the arc tube voltage becomes so high that the ballast-starter circuit can no longer supply it, the lamp goes out. The lamp will restrike after it has cooled down sufficiently. This phenomenon of alternate lighting and extinguishing of the sodium lamp is commonly known as cycling.
Cycling is a major problem in the maintenance of street lighting. The ballast-starter circuit used in conventional luminaires will burn-out if a cycling condition persists for thirty or more days. Unfortunately, it is very difficult to detect if a lamp is cycling in the field. By the time a service person has arrived at the lamp location, the lamp may have come on again. The service person will be very hesitant to remove an operating lamp, because replacement is expensive. Of course, if the cycling lamp is not replaced, the ballast-starter circuit will eventually also have to be replaced, thereby increasing the total maintenance cost of the system.
At present, a lamp replacement maintenance program serves to avoid the cycling problem. An average lamp working lifetime is determined, and the lamps are replaced before this working lifetime has expired. Since different lamps have different aging characteristics, this type of replacement program is a very expensive procedure for solving the cycling problem.
A cycling cut-off arrangement for reliably detecting the existence of a cycling malfunction in the field, and for disabling the power source from the ballast-starter circuit to thereby protect the latter from damage in the event of a cycling malfunction, has been proposed in U.S. Pat. No. 4,207,500, which is assigned to the same assignee as the present invention. Although very reliable for its intended purpose, the cycling cut-off arrangement described in said patent has not proven to be altogether satisfactory for practical reasons, because it proposed to detect the existence of a cycling malfunction by sensing the increase of voltage across the lamp. Cycling was determined when the voltage across the lamp exceeded a threshhold value.
However, this approach proposed that an additional electrical wire be connected from one of the lamp electrodes, and to be routed back to the remaining system components. This additional electrical wiring was very disadvantageous not only due to its extra material cost, but also because of the labor costs involved in retro-fitting an existing luminaire installation. Since each luminaire in a streetlighting system would require such retro-fitting, it will be appreciated that this approach is not the most practical.